On the bottomrail, remove the plugs to reveal the knotted lift cord and string ladders.

Undo the knot in the lift cord, then remove the bottomrail and slats you want to take off. Only pull the lift cord through the route holes of the slats you want to remove.

Put the bottomrail back into the ladder rung where the last slat was removed.

Cut the string above the rung that hangs below the bottomrail. Make sure that you have enough string to go through the bottom holes of the bottomrail.

String the lift cord through the bottomrail hole. Ensure that the lift cords are even so that the bottomrail will hang straight at the length you want and tie a knot. Do not attempt to change the length by pulling the lift cord.

Replace the bottomrail plugs. To avoid scratching them, use a rubber mallet or hammer.